The CDMA transmission is classified into a direct sequence (DS) method that spreads a conventionally modulated signal with a spreading code at a high rate, and a frequency hopping (FH) method. It is necessary for the FH method to divide each symbol into much smaller units called chips, and to switch the central frequency of each chip at a high rate. Since this makes it difficult to implement a device employing the FH method, the DS method is usually used.
DS-CDMA radio equipment, different from SCPC (Single Channel Per Carrier)/FDMA (Frequency Division Multiple Access) radio equipment or TDMA (Time Division Multiple Access) radio equipment, carries out at a transmitting side secondary modulation with a spreading code after data modulation, thereby transmitting a signal whose bandwidth is spread. On the other hand, at a receiving side, it restores a narrowband signal from the wideband received signal through a process called despreading, and demodulates the despread signal by a conventional method. The despreading at the receiving side requires to detect correlation between the received spreading signal and the spreading code sequence locally generated in the receiver.
Usually, it is necessary for the DS-CDMA receiver to generate a replica of the spreading code, and to synchronize the spreading code replica with the spreading code in the received signal. The synchronization process of the spreading code is divided into acquisition and tracking. Since the spreading code such as a Gold code or the like can obtain the autocorrelation only within a range of plus minus one chip, the acquisition must reduce the phase difference between the spreading code sequence in the received signal and the spreading code replica within a range much smaller than plus minus one chip, and then the tracking keeps the phase difference within this range.
The acquisition method is described, for example, in M. K. Simon, J. K. Omura, R. A. Scholtz, and B. K. Levitt, "Spread Spectrum Communications", Vol. III, Computer Science Press, 1985.
Next, the acquisition method that constitutes a prior art of the present invention will be described with reference to FIG. 1.
FIG. 1 is a block diagram of serial code acquisition process using a sliding correlator. In this figure, the reference numeral 1 designates an input terminal of the spread signal, 2 designates an output terminal for outputting a signal indicating that the acquisition has been established, 3 designates a multiplier, 5 designates an integrator/dump circuit, 6 designates a square-law detector, 7 designates a threshold decision circuit, 8 designates a digital control clock generator, and 9 designates a spreading code replica generator.
The operation of the sliding correlator as shown in FIG. 1 is as follows. To achieve the acquisition of the spreading code, the received signal input to the input terminal 1 is multiplied by the spreading code replica by the multiplier 3, and the output of the multiplier 3 is integrated for a certain time by the integrator/dump circuit 5, thereby obtaining a correlation output. The correlation output undergoes square-law detection by the square-law detector 6, and the detected output is decided whether it exceeds a threshold or not by the threshold decision circuit 7. Thus, a decision is made whether the acquisition of the spreading code has been established or not.
In a single dwell system, in which the integrator/dump circuit 5 has only one integration time (usually one symbol interval), the product of the received signal and the spreading code replica is integrated for only a dwell time.
In an actual propagation environment, however, since there are variation of the received signal level and effect of noise, acquisition errors can occur in such a manner that the acquisition fails at a true phase synchronized position, or is considered to be occurred at a wrong phase position of the spreading code replica. To reduce such errors and to increase the accuracy of the acquisition detection, it is necessary to lengthen the dwell time. The time period needed for the acquisition, however, increases with the integration time.
Generally speaking, the integration time cannot be taken long enough because the acquisition must be completed in the acquisition time period required for the system. Therefore, in an actual system, if any error acquisition is established in which the phase of the received spreading code does not coincide with that of the spreading code replica, reacquisition must be carried out because the data modulation cannot be performed correctly even if the tracking mode is started following the error acquisition. Thus, it is essential for the acquisition of the spreading code to reduce error when the acquisition is carried out on the actual propagation environment.
FIG. 2 is a block diagram showing details of the conventional sliding correlator as shown in FIG. 1, which performs the operation as described above in connection with FIG. 1. Specifically, the in-phase (I) component and the quadrature (Q) component of the spread modulated signal passed through quadrature detection are deprived of the harmonic components, and passed through an A/D converter (not shown) to be converted into digital values. The I and Q channel signals independently undergo complex multiplication by the spreading code replica, and are integrated for a certain time period. The two integrated signals are square-law detected, and then added to obtain the power of the correlation detection signal. Whether the acquisition is established or not is decided based on whether the power of the correlation detection signal exceeds the threshold or not. When the correlation detection signal is smaller than the threshold, the phase of the clock signal of the digital control clock generator 8 is advanced by one chip interval, and the correlation is detected again between the spreading code replica with its phase advanced and the spread modulated signal, followed by the threshold decision processing. The foregoing operation is repeated until the correlation detection signal (its power) exceeds the threshold value.